1. Field of the Invention
The present invention relates to a color data converting method of converting color data so that the whole color images when they are seen by apparatuses having different color reproducing ranges (color gamuts: hereinbelow, a color reproducing range is simply referred to as a xe2x80x9cgamutxe2x80x9d) are not different and, more particularly, to a color data converting method of converting color data for image display of a wide gamut into color data for printing of a narrow gamut.
2. Description of the Related Arts
Color image apparatuses such as personal computer, color printer, digital camera, and the like at present start to be widespread owing to the realization of high performance and low costs. Among them, in the color printer, it is important to print a color image displayed on a CRT display without changing an atmosphere of the whole image. In order to make the colors coincide among the different apparatuses, as a chrominance signal showing an absolute color that does not depend on the apparatus, there is a method of processing the chrominance signal by using an L*a*b* space, a CIEXYZ space, or the like of a luminance color difference separation system as a reference. For example, when an image using the color data of the L*a*b* space is outputted to a CRT display, if the color of the L*a*b* space is converted to the color data of the RGB space peculiar to the CRT display in accordance with characteristics of the CRT display and a color image is displayed, fundamentally, any CRT display can output the same color. When an image using the color data of the L*a*b* space is printed by a printer, if the color of the L*a*b* space is converted to the color of the CMY space peculiar to the printer in accordance with printer characteristics and a color image is printed, any printer can output the same color. Further, when the color data of the RGB space to be displayed on the CRT display is converted to the color data of the CMY space of the printer and a color image is printed, the RGB color data is converted into the L*a*b* color data and the L*a*b* color data is subsequently converted into the CMY color data, so that the same color image as that of a display image on the CRT display can be fundamentally printed by the printer. However, a gamut of the CRT display (hereinbelow, referred to as a display gamut) in the L*a*b* space is largely different from a gamut of the color printer (hereinbelow, referred to as a printer gamut) with respect to both of the width and shape. Many colors which can be displayed by the CRT display but cannot be printed by the color printer exist. Therefore, a color data converting method of converting the color of the CRT display which cannot be printed by the color printer into a color which can be printed is needed.
As such a color data converting method, for example, a method disclosed in JP-A-60-105376 (U.S. Pat. Ser. No. 4,675,704) has been known. FIG. 1 shows a display gamut 100 serving as an L*a*b* gamut corresponding to the RGB space of the general CRT display and a printer gamut 102 serving as an L*a*b* gamut corresponding to the CMY space of the general color printer at a vertical section of a certain hue angle value in the L*a*b* space. An (L*a*b*) value in the display gamut 100, namely, color data is shown by Oi and an (L*a*b*) value in the printer gamut 102, namely, color data is shown by Qi. In this case, i denotes an index of an integer indicative of an arbitrary space position. The printer gamut 102 in FIG. 1 is smaller than the display gamut 100 and, moreover, the positions of a white color Q1 and a black color Q2 in the printer gamut 102 are different as compared with those of a white color O1 and a black color O2 in the display gamut 100.
In the method of JP-A-60-105376, as shown in FIG. 2, as shown by arrows, saturation values of the color data O1 to O9 which are contained in the display gamut 100 and are not contained in the printer gamut 102 are reduced to saturation values in the printer gamut 102 without changing hue angle values and lightness values L*, thereby obtaining the color data Q1 to Q9 contained in the printer gamut 102. In the conventional method, however, even when saturation values a*b* are maximally compressed, for example, from the color data O1 to Q1 or from O9 to Q9, the color data is not included in the printer gamut 102, so that there is a problem that when a color image is printed by the color printer, a color image of the CRT display cannot be reproduced at a high fidelity. As an algorithm to solve the problem, there is a method disclosed in JP-A-61-288662 (U.S. Pat. Ser. No. 4,758,885). In the conventional method, as shown in FIG. 3, lightness values of the color data O1 to O5 included in the display gamut 100 are first compressed toward the center of the gamut as shown by arrows, thereby obtaining color data O11 to O15. The compression of the lightness results in that the shape of the display gamut 100 is deformed to a display gamut 104 surrounded by a broken line. Subsequently, as shown in FIG. 4, saturation values of the color data O11 to O15 are decreased and moved to the printer gamut 102 as shown by arrows, thereby obtaining the color data Q1 to Q5. In the-conventional method of FIGS. 3 and 4, the problem in the conventional method of FIG. 2 that a part of the color data on the CRT display cannot be reproduced by the color printer doesn""t occur.
The conventional color data converting methods, however, have the following problems. As for the problem in the conventional method of FIG. 2 that a part of the color data on the CRT display cannot be reproduced by the color printer, as shown in FIG. 5, after the color data Q1 and Q2 were obtained by compressing the saturation values of the color data O1 and O2, by changing the lightness values of the color data Q1 and Q2 toward the center of the printer gamut 102 as shown by arrows, color data Q11 and Q12 included in the printer gamut 102 can be obtained. According to the method, however, all of color data locating in an area 106 which is included in the display gamut 100 and in which the lightness is higher than a highest lightness point in the printer gamut 102 and in an area 108 which is included in the display gamut 100 and in which the lightness is lower than the highest lightness point in the printer gamut 102 in FIG. 6 are concentrated to one point of the color data Q11 or Q12 in the printer gamut 102 in FIG. 5, so that there is a problem that a gradation deterioration occurs.
Problems of the conventional method in FIGS. 3 and 4 will now be described. The printer gamut 102 fundamentally has a xe2x80x9c less than xe2x80x9d-character shape as shown in FIG. 1. However, in a blue region of blue, violet, and purple, the printer gamut 102 has an xe2x80x9cLxe2x80x9d-character shape as shown by a boundary 110 in FIG. 7. The bottom side portion 110 of an xe2x80x9cLxe2x80x9d-character shape is inwardly curved. In a yellow region of the printer gamut, as shown in the printer gamut 102 in FIG. 8, it has a reverse xe2x80x9cLxe2x80x9d-character shape and an upper side portion 112 of the reverse xe2x80x9cLxe2x80x9d-character shape has a shape that is inwardly curved. When the compression is performed to the xe2x80x9cLxe2x80x9d-shaped and reverse xe2x80x9cLxe2x80x9d-shaped printer gamuts 102 in FIGS. 7 and 8 in the lightness direction so as to concentrate the color data to the center of the gamut in accordance with the conventional method in FIGS. 3 and 4, as shown in FIGS. 9 and 10, areas 114 and 116 out of the display gamut 100 after completion of the compression occur in a part of the printer gamut 102. Since the colors included in the printer gamuts 114 and 116 out of the display gamut 100 are not used for printing, a printing of a better quality in which the performance of the printer is fully utilized cannot be obtained. This problem can be solved by changing a compression amount of the lightness value by the saturation value. Specifically speaking, when the compression of the lightness is performed so as to concentrate the color data in the display gamut 100 to the center of the gamut, as shown by lengths of arrows 118 in FIG. 11, as the saturation value increases, a compression ratio of the lightness is decreased. As shown in FIG. 12, the display gamut 104 after the compression completely includes the printer gamut 102. When the method of changing the compression ratio of the lightness value by the saturation value is applied to the method in FIGS. 3 and 4, however, the following problem occurs.
FIG. 13 is an enlarged diagram of low lightness portions of the display gamut 104 and printer gamut 102 when the lightness values are compressed by changing the compression ratio of the lightness value L* by the saturation value as shown in FIG. 11. When the saturation values of the color data O11 to O17 included in the display gamut 104 after the compression are compressed so as to be included in the printer gamut 102, the color data O11 to O13 is compressed to the color data Q1 to Q3 and the color data O14 to O16 is compressed to one color data Q4. Therefore, when the color data O11 to O16 distributed at regular intervals is again compressed in the saturation direction, large color differences occur among the color data O13 and the color data O14 and subsequent color data. When the color image is printed, the color differences cause discontinuity of the gradation, so that it results in a very large problem. Since the color data O14 to O16 has been compressed to the same color data Q4, there is a problem that the gradation deterioration occurs in this portion.
In the conventional method of FIGS. 3 and 4, there is also a problem in a compression function of the lightness. In the conventional method, in order to preserve the gradation in the lightness direction, a compression function of the following equation is used when the lightness is compressed.                               L          out                =                              L            2                    +                                                                      L                  4                                -                                  L                  1                                                                              L                  3                                -                                  L                  2                                                      ·                          (                                                L                  in                                -                                  L                  1                                            )                                                          (        1        )            
L1: lowest lightness value of the display gamut
L2: lowest lightness value of the printer gamut
L3: highest lightness value of the display gamut
L4: highest lightness value of the printer gamut
Lout: lightness value of color data included in the display gamut before the compression
Lin: lightness value of the color data after the compression
In the compression function, as conditions of the display gamut 100 of the general CRT display and the printer gamut 102 of the general color printer, the relation between the lightness value (lightness value of the display gamut) L1 of the color data before the conversion and the lightness value (lightness value of the printer gamut) L2 of the color data after the conversion in the case where the compression of the lightness is performed by using
L1=0.0
L2=30.0
L3=100.0
L4=90.0
is obtained, so that converting characteristics 120 of FIG. 14 are derived. From the converting characteristics 120 of FIG. 14, the gradation in the lightness direction can be preserved by the compression function of the equation (1). As compared with linear characteristics 122 in which there is no change in lightness value, however, as for the color of a low lightness in the color data before the conversion, the lightness value in the color data after the conversion remarkably rises and, as for the color of a high lightness, the lightness value decreases as a whole. Consequently, there is a problem that when a bright color image displayed by the CRT display is printed by the printer, it is darkened as a whole and, when a dark color image is printed by the printer, it is brightened as a whole.
When the above problems are summarized, according to the conventional method of FIG. 2, all of the color data included in the display gamut cannot be converted into the color data included in the printer gamut and the color image of the CRT display cannot be reproduced at a high fidelity by the printer. When it is intended to solve the problem of the reproducibility, there is a problem that the gradation deterioration occurs. In the conventional method of FIGS. 3 and 4, there is a problem that the discontinuity of the gradation and the gradation deterioration occur in the blue and yellow systems.
According to the invention, there is provided a color data converting method whereby when color data is converted between gamuts which depend on apparatuses and have different sizes, a dropout of color, a fluctuation in lightness of a whole image, discontinuity of gradation, and deterioration in gradation are not caused and a display image and a print image when they are seen are not different.
According to the invention, there is provided a color data converting method of forming second color data included in a second gamut of a second color image apparatus from first color data included in a first gamut of a first color image apparatus. Specifically speaking, there is provided a color data converting method of forming second color data included in a printer gamut (second gamut) in an L*a*b* space corresponding to a CMY space of a printing apparatus such as a color printer or the like from first color data included in a display gamut (first gamut) in the L*a*b* space corresponding to an RGB space of a color displaying apparatus such as a CRT display or the like. According to the invention, the data converting method has a virtual gamut obtaining step, a color data changing step, and a color data forming step.
In the virtual gamut obtaining step, the value of one or more of a lightness value, a saturation value, and a hue angle value of the second gamut are converted and an expanded virtual gamut including all of lightness values of at least the first gamut is derived. In the color data changing step, the value of one or more of the lightness value, saturation value, and hue angle value of the color data which is not included in the virtual gamut among the color data of the first gamut are changed, thereby obtaining third color data included in the virtual gamut. In the color data forming step, a conversion opposite to the conversion performed to the second gamut in the virtual gamut obtaining step is performed to the third color data derived in the color data changing step, thereby forming second color data. According to the color data converting method of the invention, even when the second gamut is narrower than the first gamut, all of the color data of the first gamut serving as a converting source is converted to the color data of the second gamut on the conversion destination side through the color data of the virtual gamut, so that the color data which is not color converted from the first gamut to the second gamut can be eliminated.
Specifically speaking, even when the printer gamut is narrower than the display gamut of the L*a*b* space, since all of the color data of the display gamut as a converting source is converted to the color data of the printer gamut on the conversion destination side through the color data of the virtual gamut, the color data which is not color converted from the display gamut to the printer gamut can be eliminated. In the virtual gamut obtaining step, the second gamut is converted so that the lowest lightness value of the first gamut is equal to that of the second gamut and the highest lightness value of the first gamut is equal to that of the second gamut, thereby deriving the virtual gamut. Due to the color conversion using the virtual gamut, the color data which cannot be color converted is eliminated and the white color in the first color image apparatus can be converted to the white color in the second color image apparatus. Particularly, in the case where the first color image apparatus is a CRT display and the second color image apparatus is a printer, when the white color of the CRT display is printed by the printer, a phenomenon such that ink, toner, or the like is deposited onto a paper and the paper is not seen in white can be prevented. In addition, since black in the first color image apparatus is converted to black in the second color image apparatus, the coincidence of black is also obtained in addition to the coincidence of white, gradation in achromatic color in the first color image apparatus can be reproduced by the second color image apparatus without any partial distortion. It is effective in reproduction of a color image having many achromatic colors, for example, hair in a color image or the like obtained by photographing a human being by a digital camera.
In the virtual gamut obtaining step, the second gamut is expanded only in the direction of the lightness value, thereby deriving a virtual gamut. When the virtual gamut is derived by expanding and compressing the second gamut as mentioned above, the expansion is not performed in the saturation direction but it is performed only in the lightness direction. It is, thus, possible to prevent the reduction of the saturation of the color data in the second gamut formed through the virtual gamut in the color data forming step in the case where-the first gamut is remarkably larger than the second gamut as in a case where, particularly, the first color image apparatus is a CRT display and the second color image apparatus is a color printer.
In the virtual gamut obtaining step, in the second gamut in which the saturation gamut is equal to or less than an almost intermediate predetermined lightness value between the highest lightness value and the lowest lightness value in the second gamut, the expansion is performed by increasing an expansion amount in an exponential function manner in accordance with a decrease in lightness, thereby obtaining the virtual gamut. With this method, an increase in lightness that is caused when the color data in a low lightness portion of the first gamut before the conversion is converted to the color data of the second gamut through the virtual gamut is suppressed, a situation such that the whole color image after the conversion becomes bright can be prevented and the gradation in the lightness direction can be preserved without deteriorating the gradation.
In the virtual gamut obtaining step, in the second gamut of lightness which is equal to or larger than a predetermined lightness value that is substantially an intermediate value between the highest lightness value and the lowest lightness value of the second gamut, the expansion is performed by increasing the expansion amount in an exponential function manner in accordance with the increase in lightness, thereby deriving a virtual gamut. With this method, the decrease in lightness when the color data of high lightness included in the first gamut before the conversion is converted to that in the second gamut through the virtual gamut is suppressed, it is possible to prevent that the whole color image becomes dark and the gradation in the lightness direction can be preserved without deteriorating the gradation.
The virtual gamut obtaining step has a gamut expansion restricting step of reducing the absolute value of the expansion of the lightness value as the saturation value of the color data at a location where the expansion is performed in the second gamut is larger. In this manner, in the virtual gamut obtaining step, when the virtual gamut is derived, by restricting so as to reduce the absolute value for expanding the second gamut as the saturation value is larger in the second gamut, the virtual gamut with a xe2x80x9c less than xe2x80x9d-character shape corresponding to the first gamut can be derived from the second gamut of an xe2x80x9cLxe2x80x9d-character shape in which the bottom surface is inwardly curved and the color data of the second gamut can be derived from the color data arranged at regular intervals in the first gamut through the virtual gamut without causing discontinuity of the gradation and deterioration of the gradation.
In the gamut expansion restricting step, the absolute value of the expansion of the lightness value is reduced in accordance with a function proportional to the saturation value. When the virtual gamut is derived in the virtual gamut obtaining step in this manner, by restricting in a manner such that as the saturation value is larger, the absolute value to expand the second gamut is reduced by a function proportional to the saturation value within the second gamut, the virtual gamut having the xe2x80x9c less than xe2x80x9d-character shape corresponding to the first gamut can be derived from the second gamut having an xe2x80x9cLxe2x80x9d-character shape in which the bottom surface is curved inwardly. After the color data arranged at regular intervals in the first gamut was changed to the color data of the virtual gamut, by converting the resultant data into the color data in the second gamut by using a function opposite to the function used to derive the virtual gamut, the problem that the discontinuity of gradation and the deterioration of gradation occur is also eliminated.
In the gamut expansion restricting step, an absolute value of the expansion of the lightness value is decreased in an exponential function manner in accordance with the increase in saturation value. In this manner, when the virtual gamut is derived in the virtual gamut obtaining step, by restricting so as to decrease the absolute value of the expansion of the lightness value in an exponential function manner in accordance with the increase in saturation value within the second gamut, the virtual gamut having a xe2x80x9c less than xe2x80x9d-character shape corresponding to the first gamut can be derived from the second gamut having an xe2x80x9cLxe2x80x9d-character shape in which the bottom surface is curved inwardly. After the color data arranged at regular intervals in a xe2x80x9c less than xe2x80x9d-character shape in the first gamut was changed to the gamut in the virtual gamut, by converting the resultant data into the color data in the second gamut by using a function opposite to the exponential function used to derive the virtual gamut, the problem that the discontinuity of gradation and the deterioration of gradation occur is eliminated and the saturation value can be further preserved.
According to the color data converting method of the invention, as a specific example, the first gamut is a display gamut of the RGB color space which is used in the color displaying apparatus in the L*a*b* color space. The second gamut is a printer gamut corresponding to the CMY color space which is used in the color printing apparatus in the L*a*b* color space. The virtual gamut is a gamut expanded so as to contain at least all of the lightness values of the display gamut by converting one or more of the lightness value, saturation value, and hue angle value of the printer gamut. When the color data of the RGB space to be displayed on the CRT display is converted to color data of the CMY space of the printer, the color data converting method of the invention is as follows.
First, in the virtual gamut obtaining step, an RGB/display gamut conversion table, a printer gamut/CMY conversion table, and a virtual gamut/CMY conversion table are formed. The RGB/display gamut conversion table is a conversion table for converting the color data of the RGB space into color data of the display gamut in the L*a*b* color space. The printer gamut/CMY conversion table is a conversion table for converting color data of the printer gamut in the L*a*b* color space into color data of the CMY space. Further, the virtual gamut/CMY conversion table is a conversion table for deriving the expanded virtual gamut so as to contain at least all of the lightness values of the display gamut by converting one or more of the lightness value, saturation value, hue angle value of the printer gamut, converting the color data of the virtual gamut into color data of the CMY space, and converting color data out of the virtual gamut into specific identification values out of the gamut. In the next color data changing step, after the color data of the RGB space was converted to color data of the display gamut in the L*a*b* color space by the RGB/display gamut conversion table, one or more of the lightness value, saturation value, and hue angle value of the color data which is not included in the virtual gamut in the color data of the display gamut are changed until they are not converted into the identification values out of the gamut by the virtual gamut/CMY conversion table, thereby deriving third color data included in the virtual gamut. Finally, in the color data forming step, a conversion opposite to the conversion performed to the printer gamut in the virtual gamut obtaining step is executed to the third color data derived in the color data changing step, thereby forming second color data. The second color data is converted to color data of the CMY space by the printer gamut/CMY conversion table and the converted data is outputted to the color printing apparatus. Therefore, all of the color data contained in the display gamut can be converted to color data contained in the printer gamut. When the white color of the CRT display is printed by the printer, a situation such that ink, toner, or the like is deposited onto the paper and the paper is not seen as white can be prevented. The achromatic color gradation on the CRT display can be reproduced by the printer without a deterioration of a part thereof. When a color image of the CRT display is reproduced by the printer, it is possible to guarantee the prevention of reduction of the saturation, the prevention of the change in lightness of the whole image, the prevention of deterioration of the gradation in the lightness direction, and the preservation of the saturation.
The above and other objects, features, and advantages of the present invention will become more apparent from the following detailed description with reference to the drawings.